The autoimmune hemolytic anemia (AIHA). Much of what we currently know about the biology of this disease is form work on cold-reacting RBC autoantibodies. This form of AIHA has been more amendable to study because the antigens to which cold-reacting autoantibodies bind are well characterized and the ability to establish autoantibody-producing B-cells clones by EBV-transformation facilitates the study of autoantibody structure on a molecular level. In contrast, the more common and clinically-significant syndrome of autoimmune hemolysis id due to warm-reacting RBC autoantibodies and remains poorly understood with respect to the biochemistry of the autoantigen(s) and nature of the corresponding pathologic immune response. The major obstacle in studying this disease has been the inability to obtain sufficient quantities of autoantibodies which are homogeneous in structure and function. The proposed research will utilize a recently-described bacteriophage zeta cloning system to express in E. coli the repertoire of warm-reacting anti-RBC autoantibodies from a patient with warm AIHA. The antibody molecules thus produced will be used to identify the antigenic determinants to which warm-reacting red cell autoantibodies bind and explore the heterogeneity (clonality) of immunoglobulin variable gene usage.